The invention relates generally to cryptography, and deals more particularly with a technique for rapidly decrypting data using one encryption/decryption key and re-encrypting the data using a different encryption/decryption key.
Private data is often encrypted or enciphered using an encryption algorithm and encryption/decryption key before transmission from a first site to a second site. Consequently, if an unauthorized party without the key (which is also required for decryption) intercepts the data, the party cannot understand the data even if the proper decryption algorithm is known. The data is subsequently decrypted or deciphered at the second site by an authorized party using the proper decryption algorithm and the original key. In some cases, the data must be transmitted from the second site to a third site which does not have the original key but instead has a second key. In such a case, the encrypted data received at the second site must be decrypted using the original key, and then re-encrypted using the second key and transmitted to the third site. This process is commonly called "translation". Then, the third site can decrypt the data using the second key.
There are many known techniques or algorithms for data encryption. For example, American National Standard for Information Systems (ANSI) standard X3.92-1981 defines one popular data encryption algorithm (DEA) for a data encryption standard (DES). This algorithm provides four modes of operation (depending on the level of security required): electronic codebook (ECB) mode, cipher block chaining (CBC) mode, cipher feedback (CFB) mode and output feedback (OFB) mode.
FIG. 1 illustrates the prior art CBC mode of data encryption. The CBC mode is particularly useful for encrypting a large block of data because this mode eliminates patterns caused by the encrypting and such patterns would facilitate decryption by unauthorized parties. The data to be encrypted is divided into 64 bit blocks 10a,b . . . n at a site 11. The first block is exclusive ORed (12a) with a 64 bit initialization value (IV) 14a (which is a pre-set "intermediate value", typically all zeros), and then passed to a DES encryption unit 16a. An encryption key 15a was previously loaded into the DES encryption unit 16a, and the encryption unit 16a encrypts the result of the exclusive OR unit 12a using key 15a and a variety of logical operations and permutations that constitute the encryption algorithm. DES encryption unit 16a is further defined in "Federal Information Processing Standard (FIPS) "Data Encryption Standard", by National Bureau of Standards, US Department of Commerce January, 1977, and " Data Encryption Algorithm" by ANSI, standard #X-3.92-1981. These documents are hereby incorporated by reference as part of the present disclosure. The output of DES encryption unit 16a is the encrypted form or "cipher block" of the first data block 10a. The encrypted form is transmitted to a site 18 for decryption as described below. At site 11, this first cipher block also forms the second intermediate value and is exclusive ORed (12b) with the second data block 10b. The result is processed by another DES encryption unit 16b having the same key 15a and encryption algorithm. The output of encryption unit 16b is transmitted to site 18 for decryption. At site 11, this second cipher block output from encryption unit 16b also forms the second intermediate value and is exclusive ORed (12c) with the third data block. This process is repeated serially for all encryption units 16. Thus, the encryption of the data blocks 10a,b, . . . n proceeds sequentially using multiple encryption units.
At site 18, the first encrypted block is processed by a DES decryption unit 24a using a decryption key 15b (which is identical to the key 15a used for encryption) and complimentary logical operations and permutations. DES decryption unit 24a is further described in the foregoing FIPS and ANSI documents. The output is then exclusive ORed (25a) with the same 64 bit initialization value (or pre-set intermediate value) to yield the original data block 10a. Likewise the second encrypted block is processed by an identical DES decryption unit 24b loaded with key 15b. The output of decryption unit 24b is exclusive ORed (25b) with the first encrypted data block or cipher block (which is the second intermediate value) to yield the original data block 10b. Likewise the third encrypted block is processed by an identical DES decryption unit. The output is exclusive ORed with the second encrypted data block or cipher block to yield the third original data block. This process is repeated for all remaining cipher blocks.
FIG. 2 illustrates a prior art technique for translating at a site 28 an input cipher block to another cipher block using a different key. Data blocks are encrypted at site 11 as illustrated in FIG. 1 and then sent to a buffer 29 at site 28 illustrated in FIG. 2. Then, each cipher block is processed by a single DES unit 27 (configured for decryption) loaded with the key 15b and also stored in a buffer 31 as an intermediate value for decryption of the next cipher block. The output of decryption unit 27 is exclusive ORed (32p) with the initialization value (or pre-set intermediate value for the first block). This completes the decryption of the first cipher block. Another encryption using a different key follows in the translation process. The result of exclusive OR gate 32p is exclusive ORed (36p) with another initialization value (or preset intermediate value for the first data block) from buffer 42, and then processed by DES unit 27. However, now DES unit 27 is configured for encryption and loaded with a different encryption key 43a to yield the translated cipher block. The output of encryption unit 27 is stored in buffer 42 as the second intermediate value for encryption of the next data block and transmitted to another site for decryption and use there. The decryption at this other site can use a single decryption unit loaded with a key which is identical to key 43a, exclusive OR gate and buffer.
After the first data block is encrypted by DES unit 27, the next data block is decrypted by DES unit 27, and the foregoing process is repeated using the new intermediate values. It should be noted that the prior art process illustrated in FIG. 2 requires sequential decryption and encryption steps and this takes time.
Accordingly, a general object of the present invention is to provide a faster translation process from a cipher block based on one key to a cipher block based on a different key.